[jena]

How do you figure whether a compound is R-Rectus or S-Sinister, using a Fischer projection. Everytime I try to do it I get confused.

Please Help

Thank You  

[dexangeles]

just remember that fisher projections are in such a way that the top and bottom (vertical bonds) goes in or is going away from you, while the sides (horizontl bonds) comes out (i always think of it as arms trying to hug me ).  Orient the molecule so that the lowest ranked atom points away.  If the lowest ranked is the vertical, then all you have to do is rank the other three atoms from highest to lowest and determine whether it's R or S.  If it's located on the horizontals, you can rotate the molecule around (basically the vertical atoms stay where they are, but te horizontals switch places).  By rotating the molecule around, the vertical bonds now points toward you while the horizontals point away.  this makes it easier to determine the  R or S absolute configurations.

[Winga]

Buy a set of model and try it.